The present invention relates generally to tools for investigating a borehole traversing an earth formation, and more particularly, to an apparatus and method for measuring formation density in a borehole having irregularities along the borehole wall.
Gamma-gamma well logging instruments utilizing gamma ray sources and gamma detectors for obtaining indications of the density and P.sub.e of the formation surrounding a borehole are well known. A typical such device comprises a long, rigid sonde body containing a gamma ray radioisotopic source and at least one gamma ray detector separated by depth by about forty centimeters. The sonde, when applied against the borehole wall, will span irregularities along the borehole wall, leading to inaccurate corrections for environmental effects, such as detector standoff, under rugose conditions.
It has long been recognized that the accuracy of measurements of the density of earth formations adjacent a borehole by gamma-gamma logging is greatly affected by variations in the thickness of the mudcake formed on the wall of the borehole. In conducting density and P.sub.e determinations, it has been found to be beneficial to compensate for variations in mudcake thickness by utilizing a sonde having two gamma ray detectors located at different distances from the source as may be seen with reference to U.S. Pat. Nos. 3,321,625 to J. Wahl, 3,864,569 to J. Tittman, 4,034,218 to R. Turcotte, 4,048,495 to D. Ellis, 4,661,700 to J. Holenka, and 4,958,073 to D. Ellis. In these two detector tools, it is assumed that the borehole wall is reasonably smooth. If the borehole wall is not smooth, the sonde obtains erroneous density measurements due to the fact that the source, or either of the detectors may be exposed to a different thickness of mudcake. Attempts have been made to overcome environmental effects, such as rugosity, by making explicit measurements of the size of the environmental factor causing the effect and then making explicit corrections. U.S. Pat. Nos. 4,047,027 and 4,035,639 are exemplary of previous techniques that utilize this measurement and correction approach. While the technique disclosed in these patents has its merits, it has the disadvantage of requiring the additional apparatus necessary for making an explicit measurement of source and/or detector standoff. Other attempts have been made to overcome variations in mudcake thickness by using three or more detectors as may be seen with reference to U.S. Pat. Nos. 5,390,115 and 5,530,243. In these three or more detector tools, if the borehole wall is not smooth, the sonde will not make perfect contact with the borehole wall over the entire length of the tool thereby still requiring an explicit correction for irregularities in the region of source and/or detector standoff.